The present invention relates to an analogue-to-digital converter of the series-parallel type with controlled analogue setting. The analogue-to-digital conversion is carried out conventionally by using threshold comparators to which the input signal, represented by a voltage, is applied. Normally the thresholds of the various comparators are set at regularly graduated voltages by means of a resistive network to which a reference voltage is applied. Depending on whether or not the analogue input voltage is greater than the threshold voltage with which it is compared, the bit obtained at the output is a logical 1 or a logical 0.
When using a purely parallel method of conversion, it is clear that to code a signal with a certain accuracy, for example to give an eight-bit coding, a fairly large number of comparators (about 255) are required. To save on comparators, the encoder is generally constructed in two parts establishing two groups of comparators which both operate on the same principle, with a first part supplying the higher-order or more significant bits whereas the second part supplies the lower-order or less significant bits. However, with this type of arrangement it is necessary to provide a link between the two parts of the encoder since the incoming analogue signal needs to act on both parts of the encoder, i.e. on both groups of comparators, but the way in which the second part operates is related to the result of the coding operation performed by the first part.
In prior-art systems, the link between the two parts of the encoder is formed by an operational amplifier which subtracts the already coded signal, i.e. the one which produced the higher-order bits, from the input signal so as to provide an input voltage for the comparators in the second part.
Also, the analogue signal which is to be applied to the second part of the encoder is delayed by passing it through so-called sampling gates. However, these sampling gates have to meet two conflicting requirements. The coding needs to be performed both quickly and accurately, but if accuracy is required it slows down the operation of the gates, quite apart from the matter of their cost.